The standard telephone consists of a base unit and a handset unit connected to each other by an electrical cord. The base unit itself is connected by another cord to a receptacle on a wall, telephone pole or a similar immovable structure to which the telephone network line extends. Therefore, the range of movement of the operator of the telephone is quite limited. Even when the cords connecting the handset unit to the base unit and the base unit to the wall are long, it can be cumbersome either to move the entire telephone around to make calls from different locations or to walk around with the handset unit once a call has been placed. The simple fact that there is always a continuous physical connection between the person making the phone call and the immovable wall or other fixed structure can be a great inconvenience.
The cordless telephone represents a significant improvement over the standard telephone. In the conventional cordless telephone, the base unit is still connected to the receptacle on the immovable wall or the like by a cord so that message signals from the telephone network line may be received and transmitted. However, the handset unit of the cordless telephone is an independently operative unit from which calls may be made and by which calls may be received with no physical connection to the base unit. The handset unit has a transmitting/receiving system or transceiver, a loudspeaker in the earpiece and a microphone in the mouthpiece. The base unit and the handset unit of the cordless telephone communicate with each other over a communication channel established by the transmission and reception of electromagnetic waves, conventionally radio waves. The handset unit may then be taken considerable distances from the base unit while still making and receiving telephone calls. Since there is no telephone cord extending between the handset unit and the base unit, the operator is free to move about without hindrance.
Typically there are ten duplex channels permitted for each system occupying a bandwidth set by the Federal Communications Commission. Cordless telephones using all ten duplex channels therefore may not have a separate control channel over which control information may be transmitted for determining which duplex channel will be selected for use during a particular conversation. This control information must be transmitted over one of the ten duplex channels themselves.
In order to transmit such control information without interfering with other telephone calls already in progress, a multichannel access (MCA) system has been proposed in which the cordless telephone searches for a vacant channel whenever an outgoing call is to be made or an incoming call is received and then the handset unit and base unit communicate with each other over the vacant channel. This permits a number of cordless telephones to be used simultaneously within the same general area without creating interference for each other by multiple transmissions over the same duplex channel. However, it is important to ensure that the cordless telephone does not erroneously attempt to establish communication over a channel with interference or already in use.
Although ten duplex channels are permitted for cordless telephones, many conventional cordless telephones are built with only one duplex channel available therein. As a result, if an MCA system cordless telephone identifies a vacant channel at the start of an incoming/outgoing call and attempts to establish communication thereover, another cordless telephone built to use only the identified channel may start to use the identified channel during the initial set-up procedure and may cause interference with the MCA system cordless telephone.
Furthermore, the MCA system cordless telephone will scan every channel in sequence at the start of the talk mode to identify a vacant channel and so may take a relatively long time to establish communication.